FR2729027A1 - METHOD AND APPARATUS FOR ALLOCATING COMMUNICATION PATHWAYS TO NEARBY AND NEARBY MOBILE IN AN AMRT CELLULAR RADIO SYSTEM, ESPECIALLY A GSM SYSTEM - Google Patents

Abstract

The base station performs the following steps: - analysis of the uplink frames received: a) according to a first time window (Fi) shifted by a first nominal guard shift (3.TS) corresponding to the minimum distance from nearby mobiles and b ) following a second time window (F'i) shifted by a second value shift (3.TS + DELTA'T) corresponding to the minimum distance of distant mobiles, - search, in each of these time windows, for peaks of 'autocorrelation for channel allocation request time intervals, so as to: a) on the one hand, recognize a mobile requesting the allocation of a channel and b) on the other hand, determine the near or far character of the mobile thus recognized, and allocation in the baseband of a free time interval (Ti) to each mobile thus recognized, by providing, for nearby mobiles, at least one offset time interval (TS) with respect to the frame initially issued.

Description

1i. 2729027 Method * t device for allocating control channels to

  The present invention relates to time-division multiple access (TDMA or TDMA) radio-communication cellular systems of the type AMIT cellular radiocommunication system. The European GSM system is such a typical TDMA system, and the invention will be described essentially in the context of this system, for which reference may be made to

  further details in the book 'The GSM system', written and published by Michel MOULY and Marie-Bernadette PADTET, Paris, 1992. The invention is however not limited to this particular case and applies equally well, as will be understood, to any TDMA system.

  The invention relates to the particular problem of providing satisfactory coverage in large areas, for example rural or coastal areas, typically with coverage radii greater than 35 km (this value, which is in no way limiting, being the nominal value of the maximum dimension of a GS cell <likely to be covered without the need to use specific protocols) .25 In the following, the qualifier = near will be assigned to mobiles located within this radius. nominal cell (thus at a distance between 0 and 35 km from the base station) and distant to mobiles located beyond this

  limit, that is to say beyond 35 km, typically in a radius between 35 and 70 km.

  A first method for exchanging communications with distant mobiles, which is described in the book "The GSM system" mentioned above, page 347, consists in reserving two time slots (TS, Time-Slot, in GSC terminology) per mobile instead one, so as to have a double guard time, sufficient to cover distances between

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  mobile and base station (BTS, Base Transceiver Station, in GSM terminology) much higher than 35 km. This technique has the disadvantage of dividing by two the number of mobiles that can be managed simultaneously, and requires searching the position of the network access request signal over the duration of two time slots instead of one. thus increasing the duration of the sampling and search step of the main correlation peak. Another technique, disclosed in FR-A-2 702 320 in the name of the applicant, consists in using two base stations, one to cover the area 0-35 km and the other to cover the area 35-70 km. This technique is effective and overcomes the disadvantages of the first technique. However, it results in a doubling of the number of transmitters and broadcast channels CHANNEL (BCCH, in GSM terminology), even if the traffic does not justify it, which strike all the colt of the EP-0 564 429 also addresses the problem of long-range coverage in cellular networks, proposing to operate a same base station in both the near and far domains. However, this document does not give any teaching25 on how to allocate the time intervals to mobiles which we do not know a priori if they are near or far. One of the aims of the present invention is, precisely, to propose a method for realizing the allocation of time slots of traffic to mobiles indifferently near or far, which palliates the

  disadvantages of the aforementioned known techniques, in particular by keeping the same number of time slots per mobile in a given frame, without duplicating the number of base stations and broadcast channels.

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  It will be seen moreover that the method of the invention allows a mobile, whether near or far, to be

  always recognized at its first attempt to access the network, which further increases the efficiency of the system and 5 avoids penalizing one category of mobile compared to another.

  For this purpose, the method of the invention, which is, as indicated, a method in which a base station communicates with the mobiles by a downlink frame structure and the mobiles communicate with the base station by a Rising frame structure, the mobiles being distributed in near and far mobile mobiles with respect to the base station, is characterized in that the base station performs the following steps: receiving said upstream frames; analyzing these rising frames: a) following, on the one hand, a first time window offset by a first offset with respect to the corresponding downlink frames, this first offset being a nominal guard offset corresponding to the minimum distance of the near mobiles, and b) next, on the other hand, a second time window shifted by a second offset from the corresponding downlink frames, said second offset being a value offset corresponding to the minimum distance of the far-away mobiles; search, in each of these time windows, autocorrelation peaks for time slots channel allocation request, so as to: a) firstly, to recognize a mobile requesting the allocation of a channel and b ) on the other hand, determine the near or far character of the mobile thus recognized; and allocating in the baseband a free time slot to each mobile thus recognized, providing for the near mobiles at least one offset time interval with respect to the initially transmitted upstream frame. Preferably, in the allocation step, the low-rank time slots are preferably allocated to the distant mobiles and the rank time intervals are allocated.

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  higher to nearby mobiles, so as to group mobile near and far mobile on respective series of successive time intervals. The invention also covers a base station

  implementing such a method.

  Other features and advantages of the invention will appear on reading the detailed description below.

  below an embodiment, with reference to the single appended figure, which represents a chronogram of the various temporal structures involved in the implementation of the method 1 of the invention.

  In the figure, the timing diagram a represents the basic time structure of a falling frame in a

  GSM system: a frame 10 has a plurality of time slots 12, designated 0 to 7 and carrying the information from the base station to the mobile.

  The timing diagram b represents the structure of the upstream frames, that is to say sent back from the mobiles to the base station: each of these frames 14 also comprises eight time slots 16, similar to the time slots 12 of the falling frames 10, but offset by a time interval AiT = 3.TS, where TS is the duration of a time interval. For more details on the structure and role of

  these descending and rising frames, we can refer to the aforementioned work * The GSM system.

  In a conventional GSM system, that is to say, operating only with nearby mobiles, the station

  The base station receives a continuous stream of data consisting of frames in which each time interval can correspond to a rising communication from a mobile.

  When a mobile wishes to be allocated a time slot of traffic, it sends a frame including a request for channel allocation (RA, Random Access, in GSM terminology). The base station must, on the one hand, recognize such a type of message and, on the other hand,

2729027

  determine the propagation delay (duration of

  return between the base station and the mobile) so that it can be taken into account for the temporal sequencing of the information exchanged with the mobile. This search is carried out by an autocorrelation method on the succession of bits received, this autocorrelation being effected between, on the one hand, the received signal and, on the other hand, a reference signal corresponding to a known part of the signal. received and shifted each time 1o of the duration of an additional bit, within the limit of 63 bits of offset (63 bits corresponding to a distance between base station and mobile 35 km). The first correlation peak, which is most often the main peak, corresponds to the minimum transmission time of the radio signal, the other possibly detected peaks corresponding to secondary paths, for example after reflection on natural obstacles. As understood, this technique is not applicable

only in the case of nearby mobiles.

  In the case of distant mobiles, it is necessary to use special techniques which, until now, have not led to satisfactory results in terms of efficiency and / or installation cost. The invention proposes, in order to overcome the limitations encountered so far, to simultaneously perform a double analysis of the received rising frames, one in the conventional manner and the other with a time offset corresponding substantially to a distance of 35 km, to compare the results of two analyzes and to allocate the traffic time intervals to the mobile near or far depending on the results of these analyzes. More precisely, the chronogram of line b of the figure shows the first of these analyzes, which is the classical analysis carried out by any GSM system, and on the chronogram of the line c the shifted analysis.

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  simultaneous, combined with the previous characteristic of the invention. For the sake of a better understanding of the figures, in the example illustrated, the time intervals corresponding to close mobiles are shown with simple hatching, with cross-hatching the time intervals corresponding to distant mobiles and the absence of hatching the intervals. unused time. The first analysis, not shifted, takes into account only 1o the nearby mobiles. In the example shown, we are in the presence of four nearby mobiles, which have been detected

  in the analysis windows F0, F5, F6 and F7 (where F0 is the signaling channel common to the three traffic channels F5, F6 and F7, corresponding to the three close mobiles A, B and C).

  On the line c, four distant mobiles will be identified, by an analysis carried out in windows shifted by a time interval A 'T, of duration corresponding to a distance between mobile and base of 35 km, or 63 bits: from made of this shift, nearby mobiles, whose propagation time is low, will not be

  detected while the distant motives will be detected, for example the three distant mobiles X, Y and Z detected in the windows F'l, F'2 and F'3, the window F'0 corresponding to the time interval of signaling.

  The next step, once the upstream analysis is done, is to allocate the intervals

  temporal data on the downlink frame, by recombining on the same baseband the data corresponding to the windows F0, F1, F2,... and those corresponding to the offset windows F'0, F'l, F'2, ...

  The corresponding combination is illustrated on the line d of the figure: on a falling frame 18, composed of eight time intervals 20 referenced TO, Ti, 35 ... T7, offset by the value of a time interval

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  (shift TS) with respect to the rising frames, there are successively the following time intervals: F0 signaling of the near mobiles (TO), F'0 signaling of the distant mobiles (T1), F'l traffic, F'2 and F'3 of distant mobiles X, Y and Z (T2, T3, T4), - F5, F6, F7 traffic of nearby mobiles A, B and C

(T5, T6, T7)

  Preferably (but this characteristic is not necessary) the time slots corresponding to distant mobiles and mobiles are imbricated

  close to group together the different mobiles of the same category on the same series of successive time intervals, as indicated by 22 for distant mobiles and 24 for mobile near, so as to allow optimization of the system.

  We see that we can manage on the same baseband simultaneous traffic of six mobile indifferemmnt near or far, with therefore a very small difference in efficiency20 (six mobile simultaneous instead of seven) compared to a GSM system classic only managing nearby mobiles. The invention therefore makes it possible to double the effective radius of a cell - thus to quadruple the area -

  limiting only very minimal system performance and with a very low hardware cost.

  In particular, as can be seen, the method of the invention, which is implemented downstream of the reception circuits, does not require any modification of the modulators of the base station, the only modification being in the control software. analysis, at the window of

  capturing the sampling, thus without significantly increasing the complexity of the system, including receiving equipment of the base station.

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Claims (4)

  A method for allocating time slots within communication frames with mobiles in a time-division multiple access cellular radio system, in particular a GSM system, or a base station. communicates with the mobiles via a downlink frame structure and the mobiles communicate with the base station by a rising frame structure, a system in which the mobiles are distributed in near and far motions with respect to the base station, characterized in that the base station performs the following steps: reception of the rising frames, - analysis of these rising frames: a) following, on the one hand, a first time window (Fi) shifted by a first offset with respect to the frames corresponding offsets, this first offset being a nominal value guard offset (3.TS) corresponding to the minimum distance of the mobile devices proc hes, and b) next, on the other hand, a second time window (F'i) offset by a second offset from the corresponding downlink frames, this second offset25 being a value offset (3.TS + I'T ) corresponding to the minimum distance of the distant mobiles, - search, in each of these time windows, of autocorrelation peaks for time slots of channel allocation request, so as to: a) on the one hand, recognize a mobile request the allocation of a channel and b) secondly, determine the near or far character of the mobile thus recognized, and allocation in the baseband of a free time interval (Ti) to each mobile thus recognized , providing, for the near mobile, at least one interval 9 2729027   offset time (TS) with respect to the initially transmitted rising frame. 2. The method of claim 1, wherein, in the allocation step, the low-rank time slots (T1, T2, T3, T4) are preferably allocated to the distant mobiles and the higher-ranking time slots ( TO, T7, T6, T5) to close mobiles, so as to group mobile near and far mobile on respective series of successive time intervals. 3. A base station for a time division multiple access type cellular radio system, in particular a GSM system, in which the base station communicates with the mobiles by a downlink structure and the mobiles communicate with each other. the base station by a rising frame structure, and wherein the mobiles are distributed in near and far mobile mobiles with respect to the base station, the base station including allocation means, for allocating in baseband temporal intervals within frames of communication with mobiles, base station characterized in that the allocation means comprise: - means for receiving the rising frames, - means for analyzing these rising frames: a) next on the one hand, a first time window (Fi) shifted by a first offset with respect to the corresponding descending frames, this first shift being one nominal value guard (3.TS) corresponding to the minimum distance of the near mobiles, and b) next, secondly, a second temporal window (F'i) offset by a second offset from the descending frames corresponding, this second offset being a value shift (3.TS + L 'T) corresponding to the minimum distance of the distant mobiles, 2729027   means for searching, in each of these time windows, for autocorrelation peaks for time slots of channel allocation request, so as to: a) firstly, to recognize a mobile requesting the allocation of a channel and b) on the other hand, determine the near or far character of the mobile thus recognized, and - means for allocating in the baseband a free time interval (Ti) to each mobile thus recognized, by providing, for the near mobile, at least one offset time interval (TS) with respect to the initially transmitted rising frame. 4. The base station of claim 3, wherein the allocation means preferentially allocate the low-rank time slots (T1, T2, T3, T4) to the distant mobiles and the higher rank time slots (TO, T7, T6, T5) to close mobiles, so as to group mobile near and far mobile on   respective series of successive time intervals.